An existing 3GPP LTE system provides high-speed mobile communication services each having a maximum bandwidth of 20 MHz. However, with the recent diversity of service types, transmission speeds, etc., higher-speed mobile communication systems are becoming required more often.
In order to meet such requirements, an IMT-advanced communication system has been introduced which supports a wider bandwidth (maximum 100 MHz) than existing 3GPP LTE Release-8. Carriers for mobile communication generally have a contiguous spectrum, but recently, carriers having non-contiguous spectrums are required to implement complicated functions of ITU-based user equipments (UEs). With this, requirements for reuse of carriers that are used in existing mobile communication systems are also increasing. It is possible that a spread spectrum of 20 MHz or more could be made through carrier aggregation (CA) which is a kind of communication technique for bandwidth spreading. In CA, through integration of component carriers (CCs), a wider carrier band may be ensured, and since each CC may appear as a LTE carrier to the LTE UE, compatibility with existing systems may be ensured.
However, since existing mobile communication services do not consider such non-contiguous bands, they cannot provide efficient communication methods that use non-contiguous bands of component carriers.
Also, in order for user equipment to receive a service, it is necessary to select a cell, to maintain downlink timing alignment and to establish and maintain uplink timing alignment through random access. If the user equipment accesses non-contiguous carrier sets, the operations should be performed for each CC. In this case, a case may occur where traffic congestion occurs in some CCs and enough traffic remains in other CCs. Accordingly, a base station is needed to perform call admission control (CAC) and load balancing (LB) for efficient use of resources between CCs.